Vol. 13, Special Issue 6
As I made a recent drive through Great Smoky Mountains National Park, I saw
stand after stand of dead Tsuga canadensis (L.) Carr. (Eastern Hemlock) trees scattered
across the landscape and was reminded of the profound impacts an exotic pest
can have on a susceptible host and the ecosystems that the host occupies. The same
devastating pattern of loss caused by Adelges tsugae Annand (Hemlock Woolly
Adelgid, [HWA]) can be observed in the eastern US throughout the range of Eastern
Hemlock and Tsuga caroliniana Englm. (Carolina Hemlock).
HWA was first documented in the eastern US near Richmond, VA in 1951, and
in the last ≈60 years, it has infested natural stands and horticultural plants in all
habitats where hemlocks grew. As soon as scientists became aware of the invasive
nature of HWA, researchers began trying to find ways to eradicate this pest.
However, these early efforts were largely ineffective, and HWA spread quickly with
resulting high levels of hemlock mortality. Thus, we are at a point where increased
understanding of HWA interactions and impacts as well as renewed research into
avenues for control of the infestation are critical to saving the remaining hemlock
forests of the eastern US. This special issue was proposed to help address these
needs by recounting what is known about the history and current status of the infestation,
presenting up-to-date information researchers have discovered about the
nature of the HWA insect/host interactions and the overall consequences of Eastern
Hemlock mortality on their native ecosystems, and highlighting the most promising
approaches being pursued to contain it.
To start the volume off, Preisser et al. (2014) lay out the background information
on what is currently known about the HWA infestation in the eastern US.
Abdella (2014) supplements that introductory perspective with a comprehensive
look at how HWA has impacted National Park lands and the management efforts
taken to date. Levy and Walker (2104) complement this broad focus with a
narrower examination of the decline in a single North Carolina hemlock population
caused by HWA.
Although Eastern Hemlock will occupy upland sites, the cascading effects of
stand mortality are probably most felt within the riparian zone. Loss of hemlock
forests has altered populations of plants, soil microbes, aquatic organisms, and the
animal species dependent on them, including humans. Zukswert et al. (2014), Ribbons
(2014), Brown and Weinkam (2014), Mathewson (2014), and Li et al. (2014)
touch upon a wide range of these effects and attempt to predict what is in store
should the infestation continue to spread.
As Oten et al. (2014), Jetton et al. (2014), and Fischer et al. (2014) describe,
efforts are continuing to develop methods to control HWA. Work is underway
1BCH-EPP Department, Box 9655, Mississippi State University, Mississippi State, MS
Forest Impacts and Ecosystem Effects of the Hemlock Woolly Adelgid in the Eastern US
2014 Southeastern Naturalist 13(Special Issue 6):ii–iv
2014 Vol. 13, Special Issue 6
to identify and develop HWA-resistant lines of Eastern Hemlock and Carolina
Hemlock. Some breeding programs are also investigating the efficacy of hybrid
hemlock introductions for revegetation efforts in conjunction with potential biological
control insects showing the greatest success such as Laricobius spp.
Even if HWA resistant genotypes are found and propagated for reintroduction,
many scientists predict that habitats which were once dominated by hemlocks will
never return to their original condition. The possibility that intact, native Eastern
and Carolina Hemlock stands will disappear from the landscape suggests that every
effort should be made to get baseline data from any remaining stands, especially for
understudied taxa and parameters for which data is scant or completely lacking such
as soil microbial communities. With that in mind, Baird et al. (2014a, b) document
mycorrhizal fungi found in hemlock forests. Such baseline data will increase our
understanding of forest processes and enhance restoration efforts.
The research included in this volume provides a broad sampling of the knowledge
obtained since the introduction of the Hemlock Woolly Adelgid. More researchers
are becoming involved in these studies as our understanding of the severity and
far-reaching impacts of the infestation are becoming apparent. The results of the
most recent work offer some promise that combinations of biological control and
planting resistant varieties will allow us to restore some type of Eastern Hemlock
forest to the landscape without constant chemical mitigations. One by one, pieces of
the hemlock–HWA puzzle are being added, and we are hopeful that research efforts
will help to restore Eastern and Carolina Hemlock forests. Perhaps the models that
have been developed and are now being applied to the Castanea dentata (Marsh.)
Borkh. (American Chestnut) reforestation effort can be used for Eastern Hemlock
reintroduction in the future so that they may again grace our forest landscapes.
Abella, S.R. 2014. Impacts and management of Hemlock Woolly Adelgid in national parks
of the eastern United States. Southeastern Naturalist 6:16–45.
Baird, R.B. Stokes, A. Wood-Jones, M. Alexander, C. Watson, G. Taylor, K. Johnson, T.
Remaley, and S. Diehl. 2014a. Fleshy saprophytic and ectomycorrhizal fungal communities
associated with healthy and declining Eastern Hemlock stands in Great Smoky
Mountains National Park. Southeastern Naturalist 6:192–218.
Baird, R. E. Stokes, A. Wood-Jones, C. Watson, M. Alexander, G. Taylor, K. Johnson, P.
Threadgill, and S. Diehl. 2014b. A molecular clone and culture inventory of the root fungal
community associated with Eastern Hemlock in Great Smoky Mountains National
Park. Southeastern Naturalist 6:219–237.
Brown, D.R., and T. Weinkam. 2014. Predicting bird community changes to invasion of
Hemlock Woolly Adelgid in Kentucky. Southeastern Naturalist 6:104–116.
Fischer, M.J., N.P. Havill, C.S. Jubb, S.W. Prosser, B.D. Opell, S.M. Salom, and L.T. Kok.
2014. Contamination delays the release of Laricobius osakensis for biological control
of Hemlock Woolly Adelgid: Cryptic diversity in Japanese Laricobius spp. and colonypurification
techniques. Southeastern Naturalist 6:178–191.
Vol. 13, Special Issue 6
Jetton, R., W.A. Whittier, and W.S. Dvorak. 2014.Evaluation of cold–moist stratification
treatments for germinating Eastern and Carolina Hemlock seeds for ex situ gene conservation.
Southeastern Naturalist 6:168–177.
Levy, F., and E.S. Walker. 2014. Pattern and rate of decline of apopulation of Carolina Hemlock
(Tsuga caroliniana Engelm.) in North Carolina. Southeastern Naturalist 6:46–60.
Li, X., E.L. Preisser, K.J. Boyle, T.P. Holmes, A. Liebhold, D. Orwig, and K. Moeltner.
2014.Potential social and economic impacts of the Hemlock Woolly Adelgid in southern
New England. Southeastern Naturalist 6:130–146.
Mathewson, B.G. 2014. The relative abundance of the juvenile phase of the Eastern Red-
Spotted Newt in Eastern Hemlock-dominated and mixed deciduous stands at Harvard
Forest. Southeastern Naturalist 6:117–129.
Oten, K.L.F., S.A. Merkle, R.M. Jetton, B.C. Smith, M.E. Talley, and F.P. Hain. 2014.
Understanding and developing resistance in hemlocks to the Hemlock Woolly Adelgid.
Southeastern Naturalist 6:147–167.
Preisser, E.L., K.L.F. Oten, and F.P. Hain. 2014. Hemlock Woolly Adelgid in the eastern
United States: What have we learned? Southeastern Naturalist 6:130–146.
Ribbons, R.R.. 2014. Community responses to Eastern Hemlock loss across a latitudinal
gradient. Southeastern Naturalist 6:88–103.
Zukswert, J.M., J. Bellemare, A.L. Rhodes, T. Sweezy, M. Gallogly, S. Acevedo, and R.S.
Taylor. 2014. Forest community structure differs, but not ecosystem processes, 25 Years
after Eastern Hemlock removal in an accidental experiment. Southeastern Naturalist